The outstanding development of fast personal communications like mobile phones has posed an increasing demand for semiconductor devices using Group III compound semiconductor materials such as gallium arsenide and gallium nitride. Also, compound semiconductor diodes have reached the practical utilization level and are expected to have an increasing demand because of their high emission efficiency.
These materials are generally manufactured by the vapor phase growth of organometallic compounds using reactants in the form of Group III metal alkyls having alkyl groups as a constituent component, such as trimethyl gallium, trimethyl aluminum, trimethyl indium and triethyl gallium.
The characteristics of a compound semiconductor are largely affected by the purity of a Group III metal alkyl as the starting reactant. Even traces of impurities can noticeably detract from optical and electrical characteristics. It would be desirable to have a method for preparing high purity Group III metal alkyl.
The method for preparing high purity Group III metal alkyl generally involves a distillation step. Since it is difficult to remove impurities having boiling points close to that of the Group III metal alkyl, the distillation step becomes more complex. One effective method for the removal of impurities having boiling points close to that of the Group III metal alkyl is described, for example, in JP-B 5-35154 (or WO 85/04405). This method involves forming an adduct of a Group III metal alkyl with a Group V donor ligand in a solvent, removing along with the solvent impurities that do not form the adduct, and effecting thermal dissociation, thereby recovering the Group III metal alkyl.
Undesirably, this method uses benzene, pentane or hexane as the solvent, which must be removed. Although the solvent is allegedly removed by vacuum evaporation, a long time is necessary until the solvent is completely removed from the solid by vacuum evaporation, which is disadvantageous from the industrial aspect. If an amount of the solvent is left behind, the Group III metal alkyl after thermal dissociation contains a trace of the solvent, requiring the step of separating the solvent from the Group III metal alkyl.
In the manufacturing of trimethyl gallium, the boiling point of a solvent such as benzene, pentane or hexane is close to the boiling point of trimethyl gallium. A greater burden is then imposed on the step of removing the residual solvent.
In the manufacturing of triethyl gallium, trimethyl aluminum or the like, the boiling point of the solvent used is lower than the boiling point of the Group III metal alkyl. This means that upon removal of the residual solvent by distillation, the distillation of the solvent is followed by the distillation of the Group III metal alkyl. A careful operation must be taken so as to prevent the line from being contaminated.
A similar purification method is disclosed in JP-A 62-185090. This method involves forming a coordinate compound of an alkyl gallium with a Lewis base, separating an impurity component from the coordinate compound, subjecting the coordinate compound to dissociative distillation, thereby recovering the alkyl gallium. In this method, distillation or evaporation making use of a boiling point difference is carried out for the separation of the coordinate compound and the impurity component. For providing a separating gas for concomitantly carrying away the impurity component, a method of blowing an inert gas and the utilization of an un-coordinate alkyl gallium or a lower hydrocarbon are described.
However, the method of blowing an inert gas is difficult, when the coordinate compound is solid, to completely separate the impurity component from within the solid. The utilization of an un-coordinate alkyl gallium gives rise to a loss of alkyl gallium. In the event of lower hydrocarbon, it is difficult to separate the lower hydrocarbon if left behind.
The above-referred JP-A 62-185090 also describes recrystallization for the separation of the impurity component. Since the Group III metal alkyl must be handled in an inert gas-filled equipment, the workup of crystals as by washing becomes cumbersome in a scale-up system. If low hydrocarbon is used as a solvent, the separation of the solvent becomes difficult. It is thus difficult to implement the method of this patent in practice.
With these problems taken into account, a purification method of easily removing a solvent is strongly desired for the manufacturing of a Group III metal alkyl through thermal dissociation of an alkyl adduct.